1. Field of the Invention
The present invention relates to a thin film resistor structure, and more particularly, to a thin film resistor structure integrated with a metal gate structure.
2. Description of the Prior Art
In the field of semiconductor fabrication, polysilicon material has been conventionally used to form the gates of metal-Oxide-Semiconductor Field-Effect (MOSFET) transistors. However, polysilicon materials have some drawbacks: the resistance of a polysilicon gate is higher than most of any metal materials, and the conductivity rate of the polysilicon gate is therefore lower than metal wires. In order to compensate for this disadvantage, the polysilicon gate usually undergoes a silicide process to simultaneously reduce the contact resistance and the parasitic resistance (Rp), so that the conductivity rate of the polysilicon gate is improved to an acceptable range.
It is worth noting that the polysilicon is used not only to form the gates, but also to form passive devices for mixed-mode integrated circuit devices, such as capacitors, or thin film resistors, etc. It is also well-known that the resistance of a thin film resistor made of polysilicon can be adjusted by modifying factors such as temperature or pressure during the polysilicon deposition process, or by adjusting the area, the thickness, or the concentration of implanted dopants in the polysilicon layer.
Nevertheless, in the trend to replace the polysilicon gates with metal gates, those integrated passive devices that used to be made of polysilicon are also replaced with metal materials ones. Similarly to the formation process of the active devices, passive devices such as thin film resistor are fabricated by integrating the formation of the metal layer and the dielectric layer, the photolithography process, and the etching process. It is conceivable that the integration of the metal thin film resistor process and the metal gate process, particularly a gate-first process, is more complicated, and the control of the thickness and composition of the materials in use is more difficult.
Therefore looking for a way to integrate the metal gate and the thin film resistor without increasing the process complexity and costs for such strict requirements has become an important study in this field.